1. Field of the Invention
This invention relates generally to active RC filters, and more particularly to a stop band second order active RC filter that does not use a non-inverting input to receive an input signal, and that employs both positive and negative feedback to provide an imaginary zero associated with the RC filter transfer function.
2. Description of the Prior Art
All known stop band (transfer function with an imaginary zero) second order active RC filters with one operational amplifier are based on a non-inverting operational amplifier (Op Amp) with a limited gain. A non-inverting differential amplifier (Op Amp) must employ negative feedback, in which the non-inverting input of the Op Amp is used to provide a positive gain. Using an Op Amp inverting input to provide a stop band second order active RC filter is problematic however, since it increases the complexity of the Op Amp.
Many stop band second order active RC filters are known in the art. One stop band second order active RC filter that is well known in the prior art is illustrated in FIG. 1 described more fully by Claude S. Linquist, Active Network Design, Steward and Sons, 1977, pg 586.
It is therefore both advantageous and desirable in view of the foregoing, to provide a stop band second order active RC filter architecture that does not use a non-inverting input to receive an input signal and that employs both positive and negative feedback to provide an RC filter transfer function having an imaginary zero.
The present invention is directed to a stop band second order active RC filter architecture that does not use a non-inverting input to receive an input signal, and that employs both positive and negative feedback to provide an RC transfer function having an imaginary zero.
According to one embodiment, a stop band second order active RC filter comprises a single operational amplifier (Op Amp), wherein the single Op Amp has an inverting input, a non-inverting input, and an output; and a plurality of filter elements including a plurality of resistors and a plurality of capacitors, wherein at least one predetermined filter element selected from among the plurality of resistors and plurality of capacitors is configured to provide a positive feedback path between the Op Amp output and the Op Amp inverting input, and further wherein at least one predetermined filter element selected from among the plurality of resistors and plurality of capacitors is configured to provide a negative feedback path between the Op Amp output and the Op Amp inverting input, and further wherein the Op Amp does not use the non-inverting input to provide a stop band second order active RC filter transfer function having an imaginary zero.